In the year 1990, the Joint Picture Experts Group (JPEG) ratified a first version of a standard aimed at reducing the storage space needed for digital still pictures, ISO/IEC Standard 10918-1 and ITU-T Recommendation T.81. The JPEG standard started a revolution in the capture and compression of digital images. The capture and compression of digital images enabled and enhanced many categories of products ranging from nonlinear editing stations, digital still-image cameras and portable video recording devices. These products are in part based on the compression technology defined by the JPEG standard.
The JPEG standard defines a series of operating modes and formats. One such format is a progressive stream, where the image is gradually refined to a complete image in many passes. With the propagation of JPEG, support has been needed by a variety of consumer products. One such product is the DVD player/recorder. The DVD player/recorder can decode a variety of motion-video compression standards. However, DVD player/recorders are not well suited for decoding progressive JPEG images. The lack of native support becomes further exemplified as an active push in the digital still camera product domain leads to the generation of larger and more complex JPEG images. Progressive JPEG images consume large amounts of memory in order to store intermediate coefficients and complete the image decoding process. Due to the large amounts of memory stored for progressive JPEG images, conventional systems fail to provide a feasible method for transcoding progressive JPEG bitstreams into an MPEG bitstream.
The general technique of transcoding a compressed bitstream of one format into a compressed bitstream of another format is well understood. For example, U.S. Pat. No. 6,141,447 to Elliot Linzer and Aaron Wells describes such an application. For conventional JPEG to MPEG transcoding, a picture is composed of discrete cosine transform (DCT) blocks. Each DCT block consists of a DC value and a number of runs or level pairs. When the DC value is computed, the corresponding MPEG codeword is computed and placed in the output bitstream. When a run pair codeword is decoded, the corresponding MPEG codeword for the run pair is placed in the output bitstream. The resulting bitstream can be decoded quickly by a system designed for decoding MPEG bitstreams.
Conventional methods do not adequately handle two types of bitstreams. Transcoding very large resolution JPEG images is not practical because most MPEG systems have a limit on image size. For example, a main level, high profile MPEG bitstream is limited to a predefined size. Very large resolution JPEG images generally exceed the predefined size. In addition, progressive JPEG images comprise a bitstream ordered hierarchically in a number of ways. Such hierarchy makes it difficult to transcode progressive JPEG bitstreams into an MPEG bitstream.
It would be desirable to provide a method and/or apparatus that decodes a progressive JPEG bitstream as a sequentially-predicted hybrid video bitstream.